My D800 has the same behaviour. Even if set to focus priority, it is possible to make it take pictures when it's completely out of focus. For instance, if I aim it at my jeans below the minimum focus distance of the lens attached, it is possible to trick it to take pictures by varying the location at which it aims (still below the minimum focus distance).
As Eric wrote, at some point it thinks it is in focus (and in fact the green dot pops up in the viewfinder). Although it is possible to trick it this way, I have found that in real life the autofocus is very reliable and fast. So what appears to be a bug doesn't seem to have any consequence for normal use as far as I am concerned.